1. Field of the Invention:
The present invention relates to a focusing method for use in a printer in which a magnification is determined for printing by varying a lens position and a conjugate distance.
2. Description of Prior Art:
Photographic film is of various kinds, such as 35 mm film, brownie film, 110 film, and so on. Photographic film is also classified as being either full-size film or half-size film. In order to deal with these types of photographic film, a printing apparatus is known in which the magnification for printing is varied by moving a lens and by changing the conjugate distance of an optical path. The amount of lens movement and the conjugate distance can be calculated using an AF (auto-focus) curve that is adopted in the form of equations. Such an AF curve is conventionally obtained in the following manner.
As schematically shown in FIG. 3, a negative film 60 is illuminated by a light source (not shown), and the light transmitted through the film is irradiated on a printing paper 64 through a lens 62, by which means printing is performed. A first lens formula (which expresses the relationship between the object point, the image point and the focal point) holds as follows: ##EQU1## where A is the distance between the film, S is the conjugate length, and f is the focal point of the lens.
Next, a magnification value ml is assigned. When the lens is focused with the assigned magnification ml, the following equation is obtained from Equation (1). ##EQU2##
Then, a magnification m2 is assigned. When the lens is focused with the assigned magnification m2, the following equation holds. ##EQU3## where Ap is the distance between the negative film and the lens reference point (a corrected value), A' is the distance between the lens reference point and the lens position, Sp is the distance between the negative film and the printing paper reference point (a corrected value), S' is the distance between the printing paper reference point and the printing paper position, A' ml and S' ml are A' and S' at the magnification ml, and A' m2 and S' m2 are A' and S' at the magnification m2.
A' and S' are obtained by actually measuring the distances (using, for example, the number of pulses of a pulse motor), and Ap and Sp are obtained by the calculation of Equation (1) when the lens is focused.
Since A' ml, S' ml, A' m2 and S' m2 represent distances from the reference points and are therefore known whereas the focal point f is determined by a lens, Ap and Sp can be obtained from Equations (2) and (3).
Since Ap and Sp are calculated using the above-described Equations, if either Ap or Sp is determined, the other Ap or Sp can be easily obtained from the following equation. ##EQU4##
In a normal printing apparatus, magnification as well as focusing must be accurate. Therefore, in an actual printing operation, it is essential that accurate focusing is provided when a magnification value is input.
In other words, even though it is considered that the lens is focused if the amount of blur is within a certain tolerance in terms of the circle of confusion, the magnification is varied even when the lens is focused, and focusing that has been varied by assigning a certain magnification value must therefore be corrected.
Thus, A' and S' at a certain magnification are calculated by the following procedure.
First, a second lens formula (which expresses the magnification) holds as follows: ##EQU5##
Am and Sm are obtained from the following equations as in the case where the focusing is obtained. ##EQU6##
Since Am and Sm are calculated using Equations (6) and (7), if Am or Sm is determined, the other Am or Sm can be obtained from the following equation. ##EQU7##
A' and S' are obtained from simultaneous equations in respect of Equations (4) and (8).
The above-described focusing operation is repeated so as to obtain optimum values for Ap, Sp, Am and Sm, and the thus-obtained values are stored in a memory of a control section, by which A' and S' which ensures focusing at a certain magnification m can be easily obtained by inputting that magnification.
However, in the above-described focusing method, since an operator moves the lens in the vicinity of its focal point by operating a fine adjusting screw mounted on a lens holder, the adjustment range differs for each operator. Therefore, it is difficult for the operator to move the lens to a selected optimum focused position, and the operation is inefficient. Further, it is required that focusing be visually checked by projecting an image. However, visual checking is impossible with a so-called erect type printing apparatus in which the negative film is located above the printing paper, and application of such a focusing method is therefore limited to what is called an inverted-type printing apparatus. In addition, when the image of a negative film is to be projected, accurate visual checking is impossible due to the low level of illumination of the image.